This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Calnexin (CNX) and calreticulin (CRT) are key components of the calnexin cycle, which is responsible for quality control pathways in the endoplasmic reticulum (ER) (ref 1). CNX and CRT contain an extended arm-like P-domain and a globular lectin domain, which specifically recognizes Glc1Man9GlcNAc2 glycoproteins. The tip of the P-domain is a binding site for a protein disulphide isomerase ERp57, which assists in folding of disulphide-containing proteins. Recently, we determined the molecular envelope of ERp57 using small-angle X-ray scattering and the crystal structure of the central bb` fragment (ref 2). NMR titrations and site-directed mutagenesis were used to map the calnexin interaction surface of ERp57 to the bb` domains. Recently, we have found that the P-domain of CNX/CRT binds another ER resident protein, cyclophilin B, a peptidyl-prolyl cys-trans isomerase. We obtained crystals of cyclophilin B in complex with the P-domain, which diffract to 2.8 [unreadable] resolution on a home source. We are requesting beam time to obtain higher resolution data. These studies will provide novel insights into the protein-protein interactions involved in protein folding in the endoplasmic reticulum. This is relevant to multiple human pathologies ranging from autoimmune diseases (antibody and MHC synthesis) to viral infections (ER-mediated viral entry). REFERENCES 1) Ellgaard, L. &Helenius, A. (2001) Curr Opin Cell Biol 13, 431-37. 2) Kozlov, G. et al. (2006) Structure 14, 1331-39.